外文文獻和翻譯---在多元液體氣化中改變液相大規(guī)模擴散的暫時性影響

1、<p><b>　　南京工業(yè)大學</b></p><p>　　《化學工程與工藝》專業(yè)</p><p>　　本科畢業(yè)論文(設計)</p><p><b>　　外文資料翻譯</b></p><p>　　原文名稱 Effects of temporally varying liquid-ph

2、ase mass diffusivity in multicomponent droplet gasification </p><p>　　原文作者 Huiqiang Zhang, Chung K. Law </p><p>　　原文出版 Combustion an Flame </p>&

3、lt;p>　　翻譯內(nèi)容頁碼 全文 </p><p>　　中文名稱 在多元液體氣化中改變液相大規(guī)模擴散的 </p><p>　　暫時性影響 </p><p>　　學生姓名唐柯楠 專業(yè) 化學工程與工藝 班級學號040126&l

4、t;/p><p>　　指導教師（簽字） 對譯文的評價 </p><p><b>　　技術學院</b></p><p><b>　　2008年 6 月</b></p><p>　　Effects of temporally varying liquid-phase ma

5、ss diffusivity</p><p>　　in multicomponent droplet gasification</p><p><b>　　Abstract</b></p><p>　　The relative roles of liquid-phase diffusional resistance and volatility

6、 differential in multicomponent droplet gasification are revisited, recognizing that liquid-phase mass diffusivities can be substantially increased as the droplet is progressively heated upon initiation of gasification,

7、leading to a corresponding substantial weakening of the diffusional resistance. Calculations performed using realistic and temperature-dependent thermal and mass diffusivities indeed substantiate this influen</p>

8、<p>　　Keywords: Multicomponent droplet; Liquid mass diffusivity; Distillation</p><p>　　1. Introduction</p><p>　　It is well established that the gasification mechanism of a multicomponent dr

9、oplet is controlled by three competing factors, namely the volatility differentials, the liquid-phase mass diffusivity, and the droplet surface regression rate [1–7]. Consequently, for slow surface regression relative to

10、 mass diffusion, as in the case of vaporization in a low-temperature environment, the droplet composition tends to be perpetually uniformized and the fractional gasification rates would be closely contro</p><p

11、>　　Since mass diffusion does occur in the liquid, even for situations of slow diffusion and rapid gasification, the gasification mechanism that has emerged for such mass-diffusion-limited gasification is one that cons

12、ists of three periods [3,4]. Specifically, after initiation of gasification, most of the volatile components in the surface layer are preferentially gasified, leaving this layer with a higher concentration of the less vo

13、latile, higher-boiling-point components. The droplet temperature s</p><p>　　After the concentration layer is established, the supply of species from the droplet interior to the surface is controlled by the s

14、low diffusion and droplet surface regression, resulting in a prolonged period of steady-state gasification, with the diffusion rate balancing the surface regression rate. Finally, toward the end of the droplet lifetime,

15、mass diffusion becomes efficient again, resulting in a brief period during which the more volatile components are rapidly depleted from the droplet i</p><p>　　Experiments were conducted [9] in which freely f

16、alling bicomponent droplets undergoing either pure vaporization or burning were abstracted at various stages of their lifetime, and the spatially averaged composition was subsequently analyzed. Results show that, except

17、for mixtures whose volatility differential is minimal, the average molar fraction of the volatile component steadily decreases quite substantially, implying that a steady mode of gasification is not attained. Furthermore

18、, since the </p><p>　　The primary objective of the present study is to gain further understanding, particularly quantitatively, on the gasification mechanism of multicomponent droplets, with the following fo

19、cuses. First, recognizing that diffusion-limited gasification is favored for rapid gasification rates and components with large volatility differentials, while distillationlike gasification is favored otherwise, we shall

20、 extend previous studies [4,7] to systematically demonstrate these distinguishing influences.</p><p>　　Second, it is also recognized that liquid-phase mass diffusivity is a sensitive function of temperature.

21、 Consequently it is reasonable to expect that the diffusivity could become progressively larger as the droplet is heated up upon the initiation of gasification, hence weakening the diffusional resistance responsible for

22、the quasi-steady behavior. The extent of this sensitivity needs to be assessed.</p><p>　　Third, as a corollary to the temperature sensitivity, and recognizing that the droplet temperature would increase with

23、 ambient pressure because of the corresponding increase of the liquid boiling point, it is also of interest to assess the extent to which diffusional resistance is further weakened as the ambient pressure is increased. T

24、his issue is of practical relevance because most internal combustion engines operate at elevated pressures.</p><p>　　Fourth, we shall study mixtures that are representative of both diesel and gasoline fuels,

25、 noting that while the former have smaller diffusivities because of their higher molecular weights, the diffusivities can be enhanced to a greater extent because the droplet can attain higher temperatures on account of t

26、he higher boiling points of these fuels. In contrast, gasoline fuels have low molecular weights but also lower boiling points. It is therefore not clear a priori what are the relative gasifi</p><p>　　The str

27、ucture of the paper is as follows. Since a satisfactory resolution of the above questions would require quantitatively realistic assessments, particularly in light of the sensitivity of the liquid-phase mass diffusivity

28、with temperature and the mixture constituents, we shall first extend, in Section 2, the constant (liquid-phase) property model [4] to variable properties. In Section 3 we shall study the response of bicomponent, diesel-l

29、ike mixtures with a large volatility differential und</p><p>　　2. Variable property formulation</p><p>　　The problem of interest is the spherically symmetric gasification of a droplet, initially

30、 of radiusand temperatureand consisting ofconstituents having similar liquid densities and characterized by their respective diffusive and thermodynamic properties. At timethis droplet is introduced, and ignited in the c

31、ase of combustion, in a stagnant, unbounded atmosphere. The atmosphere is characterized by its temperatureits pressureand the concentrations of itsspecies consisting of thegasifying species, </p><p>　　subjec

32、t to the initial and boundary conditions</p><p>　　whereis the droplet temperature, the liquid mass fraction of speciesthe radial coordinate, andthe droplet radius. Furthermore, andare the nondimensional expr

33、essions for the total and fractional mass vaporization rates, respectively, andare the nondimensional heat transfer to the droplet from the gas and the latent heat of vaporization, respectively, andand</p><p&g

34、t;　　whereis the thermal conductivity, the specific heat, and the subscriptsandrespectively designate the gas and liquid phases. Expressions for some of these parameters are given in [4]. It is also noted that the use of

35、Fick’s law of diffusion for the mass fractionsinstead of the molar fraction, holds rigorously for a bicomponent mixture, which is the case studied in the rest of this paper, and approximately for components with similar

36、molecular weights.</p><p>　　The above equations can be readily solved numerically, given the gas-phase conditions or solutions. The gas-phase properties are treated as constants while the liquid-phase proper

37、ties are treated as variables. This is a reasonable approximation because the transient nature of the present problem is driven by the corresponding transient variation of the liquid-phase diffusivity. The gas-phase prop

38、erties, while spatially varying, are not expected to be temporally varying to any great extent.</p><p>　　It is also noted that this is a moving boundary problem because of the regressing droplet surface. In

39、particular, although Eqs. (1) and (2) are the standard heat diffusion equations consisting of the transient and diffusion terms, the regressing surface imparts a convective influence to the transport processes within the

40、 droplet.</p><p>　　The constituents of the mixtures studied are alkanes. In Appendix A we list the relations used in the evaluation of the various liquid-phase properties of these constituents and their mixt

41、ures.</p><p>　　3. Constant-property results</p><p>　　It is useful to first specialize the variable liquidphase property formulation to that of constant properties in order to investigate the rol

42、es of volatility differential and surface regression rate in the gasification process. Two cases are considered: an equimolar hexadecane–tetradecane droplet burning in 1300-K, 1-atm air, and an equimolar hexadecane– deca

43、ne droplet undergoing vaporization in 1020-K, 1-atm air. The former tends to promote quasi-steady diffusion-limited gasification behavior b</p><p>　　The effects of liquid-phase diffusional resistance are rep

44、resented by a constant liquid-phase Lewis number, Le, defined as the ratio of a thermal diffusivity to a mass diffusivity. Various Lewis numbers are used to simulate the influence of liquid-phase diffusional resistance:

45、the larger the Le, the stronger the resistance. Since liquid-phase mass diffusivity is usually much smaller than thermal diffusivity, Le is a large number and we have adopted the value of 30, used in [4], to investigate

46、th</p><p>　　Fig. 1 shows the surface and center values of the molar fraction of the more volatile component, tetradecane, in the hexadecane–tetradecane droplet. The time used here is a nondimensionaltime, wh

47、ich is a normalized physical time when theholds rigorously. Results for the Le =30 case demonstrate that the gasification process basically follows diffusion-limited behavior in the development of a surface concentration

48、 boundary layer that persists until almost the end of the droplet lifetime, as shown </p><p>　　Fig. 2 shows the corresponding plot for the hexadecane– decane droplet undergoing vaporization. It is seen that,

49、 for the Le =30 case, the strength of the diffusional resistance is mostly maintained, except that the diffusion wave does reach the droplet center earlier, hence slightly changing the composition there. The larger volat

50、ility differential also leads to a smaller volatile concentration at the surface, as compared to the tetradecane concentration in Fig. 1. For Le =1 and 0.1, the larger </p><p>　　Fig. 3 compares the experimen

51、tal [9] and calculated temporal variations of the spatially averaged molar fraction of decane for a hexadecane–decane droplet; note that the initial concentration of the droplet used in the computation is the experimenta

52、l value. It is seen that during the initial stage, whenthe calculated values obtained by assuming the two extreme limits of diffusion, Le =30 and 0.1, agree well not only with each other but also with the experimental va

53、lue. The result for the Le =1 c</p><p>　　Subsequent to this initial period of adjustment, the calculated result for the Le =30 curve gradually levels off while that for the Le =0.1 curve decreases rapidly. T

54、he experimental result is situated somewhere in between these two limiting modes of gasification, indicating that the gasification mechanism is a mixed one.</p><p>　　In order to understand the cause for the

55、observed mixed mode behavior, we have reevaluated the temporal and spatial variations of Le based on the calculated local composition and temperature for the Le =30 case. Fig. 4 shows that there is significant variation

56、in Le, especially during the initial, transient period when the concentration boundary layer is established and the droplet is rapidly heated. After this period, the surface and center values not only progressively appro

57、ach each other, but</p><p>　　The cause for this substantial and rapid reduction of Le is the equally substantial and rapid increase in the droplet temperature and the sensitivity with which liquid-phase mass

58、 diffusivity varies with temperature. To appreciate this influence, we also plot in Fig. 4 the temporal variation of the droplet temperature profile. It is then seen that the entire droplet temperature, from the center t

59、o the surface, increases substantially and rapidly during the same period of the substantial variation</p><p>　　In view of the importance of Le in the droplet gasification mechanism, and that it varies subst

60、antially and sensitively with the droplet temperature, a quantitatively realistic study of the droplet gasification process must involve variable liquid-phase properties, to be presented next.</p><p>　　4. Va

61、lidation of variable-property formulation</p><p>　　We first validate our variable-property formulation and calculation by comparing the calculated results with the literature experimental data [9]. Fig. 5 sh

62、ows the comparison for the temporal variations of the average concentration of the volatile component, for droplets of hexadecane (designated as C16) with tetradecane (C14), dodecane (C12), and decane (C10) undergoing pu

63、re vaporization in the 1020-K, 1-atm environment. The comparison can be considered to be quantitatively satisfactory. On the</p><p>　　Recognizing that errors could cancel out when quantities are compared on

64、a normalized basis, as shown for Fig. 5, we next compare in Fig. 6 the temporal variations of the actual constituent volumes for these mixtures. The same degree of agreement is again observed. In particular, it is seen t

65、hat there exists an initial period during which the less volatile component practically does not gasify, while the content of the more volatile component continuously diminishes. This is consistent with the </p>&

66、lt;p>　　Comparisons have also been conducted for all the experimental data of [9], which include burning cases as well as other mixtures. The same degree of agreement was observed.</p><p>　　Having validate

67、d our variable property formulation, we now proceed to study some specific issues in multicomponent droplet gasification.</p><p>　　5. Variable-property results</p><p>　　5.1. Effects of ambient t

68、emperature</p><p>　　A key factor influencing the efficiency of diffusional resistance is the droplet surface regression rate. For a droplet undergoing pure vaporization, the controlling system parameter affe

69、cting the regression rate is the ambient temperature. Consequently, in Fig. 7, we have plotted the average molar fraction of decane in a hexadecane–decane droplet undergoing vaporization in ambient temperatures of 1500,

70、1000, and 500 K. The respective batch distillation limits simulated using Le = 0.1 are also i</p><p>　　5.2. Effects of ambient pressure</p><p>　　We shall again use the hexadecane–decane mixture

71、for demonstration. As mentioned earlier, since the attainable droplet temperature is expected to increase with increasing ambient pressure, and since liquid-phase mass diffusivity increases sensitively with increasing te

72、mperature, we expect that diffusion will be facilitated with increasing pressure, causing the gasification to be more distillation-like. Fig. 8 shows that the droplet temperature indeed increases substantially as the amb

73、ient pressu</p><p>　　To assess the influence of ambient pressure on Le, Fig. 9 shows that the mass diffusivity indeed increases substantially with increasing pressure, while the change of the thermal diffusi

74、vity is relatively minor. Consequently Fig. 10 shows that Le decreases substantially with increasing pressure, in the same manner as in Fig. 4. In particular, Le attains an approximately constant value of only 3 at 15 at

75、 mospheres, which is to be contrasted with the value of about 10 at 1 atmosphere.</p><p>　　Fig. 11 shows the temporal variation of the surface and center concentrations of decane. It is seen that, with the f

76、acilitated diffusive transport, the diffusion wave reaches the droplet center at an earlier time. Furthermore, it also facilitates transport of the volatile component to the surface layer, thereby leading to a higher con

77、centration at the surface as pressure is increased. Fig. 12 shows the temporal variation of the average concentration of decane, together with the values for the di</p><p>　　The reduced sensitivity of the vo

78、latility differential with increasing pressure is also responsible for the slower rate of depletion of the volatile component in the distillation limit, leading to the observed behavior for the Le = 0.1 curves in Fig. 12

79、. In general, Fig. 12 shows that gasification becomes more distillation-like with increasing pressure.</p><p>　　5.3. Gasification of lighter fuels</p><p>　　In the above we have studied the gasif

80、ication of fuels whose volatility is relevant to diesel fuels. We now assess the effects of diffusional resistance on fuels relevant to gasoline fuels. These fuels have lower molecular weights, which tend to render them

81、more diffusive. On the other hand, they have lower boiling points and critical temperatures, which tend to render them less diffusive. It is therefore not clear a priori which factor has the stronger influence.</p>

82、<p>　　We have selected decane and heptane as representative fuels bracketing the volatility of gasoline fuels. Fig. 13 shows that the attainable values of Le are higher than those of the diesel-like fuels, shown i

83、n Fig. 10, suggesting that the lower droplet temperature, instead of the smaller molecular weight, has a stronger influence on the diffusional resistance. Indeed, this is substantiated by the overall lower rate of deplet

84、ion of the volatile component, heptane, shown in Fig. 14 for different pr</p><p>　　6. Concluding remarks</p><p>　　Our understanding of multicomponent droplet gasification has now come full circl

85、e. The role of mass diffusion in the early stage of modeling not being recognized, the gasification mechanism was based on batch distillation, being controlled solely by the volatility differentials of the constituents.

86、Subsequently it was recognized that liquid-phase mass diffusivity is very low, and as such would lead to a completely different gasification mechanism, with the volatility differential having a minor,</p><p>

87、;　　Acknowledgment</p><p>　　This work was supported by the U.S. Air Force Office of Scientific Research under the technical monitoring of Dr. Mitat Birkan.</p><p>　　Appendix A. Specifications of

88、property values</p><p>　　Since variation of the properties is an essential feature of the present investigation, we list herein the relations used in their evaluation.</p><p><b>　　Density&

89、lt;/b></p><p>　　The change of density with temperature and pressure were obtained from the NIST database: http:// webbook.nist.gov/chemistry/form-ser.html.</p><p>　　Heat capacity</p>&

90、lt;p>　　Expressions and values for Cp were obtained from Ref. [10] as follows. For a pure liquid, Cp is given by</p><p>　　The parameters A, B, and C for the different liquids studied are listed in Table 1.

91、</p><p>　　The for a bicomponent mixture consisting of and is </p><p>　　Thermal conductivity</p><p>　　Latini’s method [11] was adopted to calculate the thermal conductivity of a pur

92、e liquid,</p><p><b>　　where</b></p><p>　　is the boiling temperature at 1 atm, the critical temperature, the molecular weight, and</p><p>　　For hydrocarbons, and The b

93、oiling point, critical temperature, and molecular weight for heptane, decane, dodecane, tetradecane, and hexadecane are listed in Table 2. For a bicomponent mixture of and we have [10]</p><p>　　The componen

94、ts are chosen so that </p><p><b>　　Viscosity</b></p><p>　　The liquid viscosity is given by [12]</p><p>　　The parameters and for the different liquid components are list

95、ed in Table 3.</p><p>　　Mass diffusion coefficient</p><p>　　The mass diffusion coefficients for a bicomponent mixture of and </p><p>　　where represents the mutual diffusion coeffic

96、ient of solute at very low concentrations in solvent given by [10]</p><p>　　with the unit of The parameters and are the molar volume of component and at their respective normal boiling temperatures. Fur

97、thermore, for a bicomponent mixture consisting of any two of the alkanes studied herein, the adjustable parameters take their values as [13]</p><p>　　The UNIFAC volume and surface parameters and are given

98、 in Table 4.</p><p>　　Thermal diffusivity and Lewis number</p><p>　　After the above properties are obtained, the thermal diffusivity and Lewis number are respectively given by</p><p&g

99、t;　　References</p><p>　　[1] C.K. Law, Prog. Energy Combust. Sci. 8 (1982) 171–201.</p><p>　　[2] W.A. Sirignano, Fluid Dynamics and Transport of Droplets and Sprays, Cambridge Univ. Press, 1999.&